The invention relates to a printing form, specifically a printing plate, with impression sites made out of plastic.
Printing is the multiplication of information in the form of dots in a screen, characters, or solid black and white, color-spot, or multicolor areas. One objective is the highest possible quality of reproduction at an acceptable expense. Overall quality criteria that are independent of what printing method is employed are the precision of contours and graphics elements, the uniformity and density of the dots or characters, the uniformity of the screen and the maintenance of the white spaces between the dots in screen-printed solid areas, and uniform coverage in solid areas. Precise color-register tolerance is also significant in multicolor printing, and generally the uniform spread of the ink over the surface of the plate and over time.
The quality of newspaper printing is essentially dictated by the demands of the market, reader habits, the needs of advertisers, the competition, and the technical potential in relation to economics. Offset printing generally results in higher quality, although material developments and the use of photopolymer printing plates have also led to progress in letterpress newspaper printing.
Letterpress, however, demands special and relatively expensive efforts to attain the average quality of offset.
The current versions of letterpress use inks that contain minerals and vegetable materials and differ in viscosity and consistency, with their properties finally being determined by the material being printed, coverage requirements, ink absorption and bleed-through, drying properties, and smear resistance. A wide range of inking devices, from simple lifting devices through overshot fountains to the up-to-date anilox system, are employed to work the ink, control its flow, and ink the printing plate. This wide range, however, results in differences that, from the aspect of design, necessitate a wide selection of very different inks. Since the level of squeeze necessary to attain satisfactory reproduction can only be decided page by page during the printing process and the ratio of space to printed matter cannot be controlled, saturation of the printing form with high-density ink unavoidably leads to oversaturation of the components with lighter elements in the sequence of intervals along the printing flow.
Newspaper-letterpress printing occurs when the impression cylinder and the plate cylinder come into contact. The hardness requirements for the surface of the impression cylinder vary widely. The printing plate can be secured in place magnetically or mechanically. The pressure in the nip between the printing cylinder and the plate cylinder is linear over the width of the cylinders and can be linearly controlled only to the right or to the left.
Since all the impression sites on a letterpress plate are in the same plane, the dynamic pressure on both the printing areas and the empty spaces is equally powerful, necessitating make ready. Negative elements must be considerably reduced, for example, in order to some extent to attain enough pressure to ensure more or less satisfactory coverage of the material being printed.
All of these additional measures are comparatively time-consuming and are in the final analysis of only limited use. Maintaining the lights, keeping the dots open in the depths, and ensuring the fidelity to the original of linear elements even when they are covered up in the printing plate, are practically impossible.